Preheat the oven to 190C/375F/Gas 5. Line a baking tray with greaseproof paper.
Cream the butter and sugar together in a bowl until pale, light and fluffy.
Beat in the egg and vanilla extract, a little at a time, until well combined.
Stir in the flour until the mixture comes together as a dough.
Roll the dough out on a lightly floured work surface to a thickness of 1cm/½in.
Using biscuit cutters or a glass, cut biscuits out of the dough and carefully place onto the baking tray. To make into Christmas tree decorations, carefully make a hole in the top of the biscuit using a straw.
Bake the biscuits for 8-10 minutes, or until pale golden-brown. Set aside to harden for 5 minutes, then cool on a wire rack.
For the icing, sift the icing sugar into a large mixing bowl and stir in enough water to create a smooth mixture. Stir in the food colouring.
Carefully spread the icing onto the biscuits using a knife and sprinkle over the glitter. Set aside until the icing hardens.
To make honey biscuits, swap the caster sugar for 70g/2½oz honey and 30g/1oz caster sugar. Increase the vanilla extract to 2 teaspoons. Cook at 180C/Fan 160/Gas 4. Pipe on Christmas shapes with 40g/1½oz melted white chocolate instead of using icing.
For reduced sugar biscuits, use 60g/2½oz soft brown muscovado sugar instead of 100g/3½oz caster sugar. Increase the vanilla extract to 2 teaspoons. Add 1 teaspoons ground cinnamon and 1 tbsp finely chopped candied peel with the flour. Decorate with halved blanched almonds instead of icing.